In this project, we focused our studies on the genetic basis both of poxvirus virulence and of host resistance to virus infection. The acquired knowledge should contribute toward development of safe, effective recombinant vaccinia virus vaccines for animal and human use. Molluscum contagiosum virus infects a large number of HIV-1 patients as an opportunistic infection. The biology of the virus is little understood because it has not been propagated under experimental conditions until now. We have observed virus replication in 25% of the infected human foreskin grafts to the nude mouse. This may provide a model for evaluating therapeutics for treatment of this disease. The genetic basis for ectromelia virus' narrow host range is poorly understood. Analyses of the tissue tropism genes CHO, K1L and C7L revealed that the CHO gene is inactive and the K1L and C7L genes have only partial activity. Deletion of the K1L gene had no detectable effect on ectromelia virus pathogenesis in the mouse. These observations are consistent with the narrow host-range of ectromelia virus. A newly identified EV ORF predicted to encode a 28kDa protein was found to replicate normally in tested tissue culture cell lines, but replicated to levels at least 100-fold less than WT EV in the foot-pad of the mouse, spread more slowly from the primary site of inoculation to internal organs and had an LD50 that was 1000 times higher than the WT virus. This gene appears to be an important poxvirus virulence determinant in the mousepox model. The popliteal lymph node which drains the site of virus infection (by the foot-pad route) shows high levels of virus infectivity. Electron microscopy, light level histology and in situ hybridization support the model that the virus replicates in this tissue only transiently up until around 4 days p.i., after which time the lymph node is refractile to infection. The infectivity that is detected in the lymph node is most likely virus which has been carried as free virus in the lymph or in cells trafficking out of the tissue.